Abstract
In this work, the effect of sequence addition of silica and chromium precursors of one-pot synthesis of Cr/MCM-41catalysts on their structural properties and catalytic performance for oxidative dehydrogenation of ethane (ODH) to ethylene using CO2 was investigated. The developed catalysts have been characterized by several techniques such as N2 sorption analyzer, XRD, TEM, SEM, UV–Vis, FTIR, TPD, and XPS. Results showed that the pore and surface characteristics of the developed catalysts strongly depend on the employed sequence. Catalyst prepared by sequence of addition of tetraethyl orthosilicate (TEOS) then Cr, exhibited similar catalytic property to the one prepared using conventional impregnation method. On the other hand, prepared catalyst with sequence of Cr then TEOS, exhibited the highest catalytic ethylene conversion and yield ~58% and 53%, respectively at 700 °C. The enhanced performance arose from the small sizes of mesoporous silica nanoparticles that promoted the high degree of dispersion for CrOx species into meso-channels and surface of porous silica support. Moreover, those small sizes of mesoporous silica nanoparticles have enhanced the interaction of reactants with CrOx species, thus facilitating a faster reduction-oxidation cycle. Various chromium species including Cr6+, Cr3+ and crystalline Cr2O3 were found in all prepared samples; however the highest Cr6+/Cr3+ ratio, that reportedly to be highly active one, was recorded for 8 wt%Cr/MCM-41-Cr-TEOS catalyst. Furthermore, the impact of different Cr loading contents based on Cr-TEOS sequence of addition on the conversion of ethane, yield and selectivity to ethylene has been investigated as well.
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